547 research outputs found
Decoherence scenarios from micro- to macroscopic superpositions
Environment induced decoherence entails the absence of quantum interference
phenomena from the macroworld. The loss of coherence between superposed wave
packets depends on their separation. The precise temporal course depends on the
relative size of the time scales for decoherence and other processes taking
place in the open system and its environment. We use the exactly solvable model
of an harmonic oscillator coupled to a bath of oscillators to illustrate
various decoherence scenarios: These range from exponential golden-rule decay
for microscopic superpositions, system-specific decay for larger separations in
a crossover regime, and finally universal interaction-dominated decoherence for
ever more macroscopic superpositions.Comment: 11 pages, 7 figures, accompanying paper to quant-ph/020412
Decoherence and Initial Correlations in Quantum Brownian Motion
We analyze the evolution of a quantum Brownian particle starting from an
initial state that contains correlations between this system and its
environment. Using a path integral approach, we obtain a master equation for
the reduced density matrix of the system finding relatively simple expressions
for its time dependent coefficients. We examine the evolution of delocalized
initial states (Schr\"odinger's cats) investigating the effectiveness of the
decoherence process. Analytic results are obtained for an ohmic environment
(Drude's model) at zero temperature.Comment: 15 pages, RevTex, 5 figures included. Submitted to Phys. Rev.
Uniformly Accelerated Charge in a Quantum Field: From Radiation Reaction to Unruh Effect
We present a stochastic theory for the nonequilibrium dynamics of charges
moving in a quantum scalar field based on the worldline influence functional
and the close-time-path (CTP or in-in) coarse-grained effective action method.
We summarize (1) the steps leading to a derivation of a modified
Abraham-Lorentz-Dirac equation whose solutions describe a causal semiclassical
theory free of runaway solutions and without pre-acceleration patholigies, and
(2) the transformation to a stochastic effective action which generates
Abraham-Lorentz-Dirac-Langevin equations depicting the fluctuations of a
particle's worldline around its semiclassical trajectory. We point out the
misconceptions in trying to directly relate radiation reaction to vacuum
fluctuations, and discuss how, in the framework that we have developed, an
array of phenomena, from classical radiation and radiation reaction to the
Unruh effect, are interrelated to each other as manifestations at the
classical, stochastic and quantum levels. Using this method we give a
derivation of the Unruh effect for the spacetime worldline coordinates of an
accelerating charge. Our stochastic particle-field model, which was inspired by
earlier work in cosmological backreaction, can be used as an analog to the
black hole backreaction problem describing the stochastic dynamics of a black
hole event horizon.Comment: Invited talk given by BLH at the International Assembly on
Relativistic Dynamics (IARD), June 2004, Saas Fee, Switzerland. 19 pages, 1
figur
Dressed States Approach to Quantum Systems
Using the non-perturbative method of {\it dressed} states previously
introduced in JPhysA, we study effects of the environment on a quantum
mechanical system, in the case the environment is modeled by an ensemble of non
interacting harmonic oscillators. This method allows to separate the whole
system into the {\it dressed} mechanical system and the {\it dressed}
environment, in terms of which an exact, non-perturbative approach is possible.
When applied to the Brownian motion, we give explicit non-perturbative formulas
for the classical path of the particle in the weak and strong coupling regimes.
When applied to study atomic behaviours in cavities, the method accounts very
precisely for experimentally observed inhibition of atomic decay in small
cavities PhysLA, physics0111042
Environment-Induced Decoherence and the Transition From Quantum to Classical
We study dynamics of quantum open systems, paying special attention to those
aspects of their evolution which are relevant to the transition from quantum to
classical. We begin with a discussion of the conditional dynamics of simple
systems. The resulting models are straightforward but suffice to illustrate
basic physical ideas behind quantum measurements and decoherence. To discuss
decoherence and environment-induced superselection einselection in a more
general setting, we sketch perturbative as well as exact derivations of several
master equations valid for various systems. Using these equations we study
einselection employing the general strategy of the predictability sieve.
Assumptions that are usually made in the discussion of decoherence are
critically reexamined along with the ``standard lore'' to which they lead.
Restoration of quantum-classical correspondence in systems that are classically
chaotic is discussed. The dynamical second law -it is shown- can be traced to
the same phenomena that allow for the restoration of the correspondence
principle in decohering chaotic systems (where it is otherwise lost on a very
short time-scale). Quantum error correction is discussed as an example of an
anti-decoherence strategy. Implications of decoherence and einselection for the
interpretation of quantum theory are briefly pointed out.Comment: 80 pages, 7 figures included, Lectures given by both authors at the
72nd Les Houches Summer School on "Coherent Matter Waves", July-August 199
Decoherence, irreversibility and the selection by decoherence of quantum states with definite probabilities
The problem investigated in this paper is einselection, i. e. the selection
of mutually exclusive quantum states with definite probabilities through
decoherence. Its study is based on a theory of decoherence resulting from the
projection method in the quantum theory of irreversible processes, which is
general enough for giving reliable predictions. This approach leads to a
definition (or redefinition) of the coupling with the environment involving
only fluctuations. The range of application of perturbation calculus is then
wide, resulting in a rather general master equation.
Two distinct cases of decoherence are then found: (i) A ``degenerate'' case
(already encountered with solvable models) where decoherence amounts
essentially to approximate diagonalization; (ii) A general case where the
einselected states are essentially classical. They are mixed states. Their
density operators are proportional to microlocal projection operators (or
``quasi projectors'') which were previously introduced in the quantum
expression of classical properties.
It is found at various places that the main limitation in our understanding
of decoherence is the lack of a systematic method for constructing collective
observables.Comment: 54 page
Measurement of W Polarisation at LEP
The three different helicity states of W bosons produced in the reaction e+
e- -> W+ W- -> l nu q q~ at LEP are studied using leptonic and hadronic W
decays. Data at centre-of-mass energies \sqrt s = 183-209 GeV are used to
measure the polarisation of W bosons, and its dependence on the W boson
production angle. The fraction of longitudinally polarised W bosons is measured
to be 0.218 \pm 0.027 \pm 0.016 where the first uncertainty is statistical and
the second systematic, in agreement with the Standard Model expectation
Search for Anomalous Couplings in the Higgs Sector at LEP
Anomalous couplings of the Higgs boson are searched for through the processes
e^+ e^- -> H gamma, e^+ e^- -> e^+ e^- H and e^+ e^- -> HZ. The mass range 70
GeV < m_H < 190 GeV is explored using 602 pb^-1 of integrated luminosity
collected with the L3 detector at LEP at centre-of-mass energies
sqrt(s)=189-209 GeV. The Higgs decay channels H -> ffbar, H -> gamma gamma, H
-> Z\gamma and H -> WW^(*) are considered and no evidence is found for
anomalous Higgs production or decay. Limits on the anomalous couplings d, db,
Delta(g1z), Delta(kappa_gamma) and xi^2 are derived as well as limits on the H
-> gamma gamma and H -> Z gamma decay rates
Measurement of W Polarisation at LEP
The three different helicity states of W bosons produced in the reaction e+
e- -> W+ W- -> l nu q q~ at LEP are studied using leptonic and hadronic W
decays. Data at centre-of-mass energies \sqrt s = 183-209 GeV are used to
measure the polarisation of W bosons, and its dependence on the W boson
production angle. The fraction of longitudinally polarised W bosons is measured
to be 0.218 \pm 0.027 \pm 0.016 where the first uncertainty is statistical and
the second systematic, in agreement with the Standard Model expectation
Bose-Einstein Correlations of Neutral and Charged Pions in Hadronic Z Decays
Bose-Einstein correlations of both neutral and like-sign charged pion pairs
are measured in a sample of 2 million hadronic Z decays collected with the L3
detector at LEP. The analysis is performed in the four-momentum difference
range 300 MeV < Q < 2 GeV. The radius of the neutral pion source is found to be
smaller than that of charged pions. This result is in qualitative agreement
with the string fragmentation model
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